The present invention relates generally to phase change memory devices and, more particularly, to composite films for phase change memory devices.
Phase change technology is promising for next generation memory devices. It uses chalcogenide semiconductors for storing states and digital information. The chalcogenide semiconductors, also called phase change materials, have a crystalline state and an amorphous state. In the crystalline state, the phase change materials have low resistivity; while in the amorphous state, they have high resistivity. The resistivity ratios of the phase change materials in the amorphous and crystalline states are typically greater than 1000:1, and thus the phase change memory devices are unlikely to have errors for reading states. The chalcogenide semiconductors are stable at a certain temperature range in both crystalline and amorphous states and can be switched back and forth between the two states by electric pulses.
Typically, a phase change memory device is formed by placing a phase change material between two electrodes. Write operations, also called programming operations, which apply electric pulses to the memory device, and read operations, which measure the resistance of the phase change memory, are performed through the two electrodes. Generally, write operations utilize a set pulse and a reset pulse. The set pulse heats the phase change material to a temperature higher than a crystallization temperature, but below a melting temperature, for a time longer than the required crystalline time, for the crystallization to take place. The reset pulse, which turns the phase change material into an amorphous state, heats the phase change material to a temperature higher than the melting temperature. The temperature is then quickly dropped below the crystallization temperature for a time period short enough to reduce or prevent the crystallization. The phase change material is heated by controlling the current flowing through a conductive material, commonly referred to as a “heater.” The heater comprises a conductive material that, due to its resistive properties, heats up when a sufficiently high voltage differential is applied.
A protective layer is often placed over the chalcogenide material to protect the material and to protect surrounding material from damage caused by changing the state of the chalcogenide material. This protective layer may be formed by a high temperature or a low temperature process. In the high temperature process, however temperatures typically exceeding about 300° C. may cause the underlying chalcogenide material to outgass. This outgassing causes cracks or voids to develop at the interface between the chalcogenide material and the protective layer leading to device failure. On the other hand, in the low temperature process, the resultant protective layer formed over the chalcogenide material may result in poor film conformity leading, again to the development of cracks or voids at the interface between the chalcogenide material and the protective layer.
For these reasons and other reasons that will become apparent upon reading the following detailed description, there is a need for an improved phase change memory device and a method of manufacture that avoids the formation of cracks/voids associated with conventional methods.